<!DOCTYPE html>
<html>
    <head>
        <style>
            main{
                position: relative;
                border: 5px solid black;

            }

            .left-side{
                position: absolute;
                left: 0;
                top:0;

                width: 100px;
                height: 200px;
                background-color: red;
            }

            .main{
                position: absolute;
                background-color: rgb(0, 195, 255);
                left: 100px;
                right: 100px;
                height: 200px;
            }

            .right-side{
                position: absolute;
                right: 0;
                top:0;

                width: 100px;
                height: 200px;
                background-color: green;
            }
        </style>
    </head>
    <body>
        <!-- 中间自适应，两边定宽的三列布局 -->
        <main class="container">
            <div class="left-side"></div>
            <div class="main"></div>
            <div class="right-side"></div>
        </main>
    </body>
</html>